Low Temperature BI-CMOS Compatible Process For MEMS RF Resonators and Filters

ABSTRACT

A microelectromechanical system (MEMS) resonator or filter including a first conductive layer, one or more electrodes patterned in the first conductive layer which serve the function of signal input, signal output, or DC biasing, or some combination of these functions, an evacuated cavity, a resonating member comprised of a lower conductive layer and an upper structural layer, a first air gap between the resonating member and one or more of the electrodes, an upper membrane covering the cavity, and a second air gap between the resonating member and the upper membrane.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. application Ser.No. 10/316,254, filed on Dec. 10, 2002, which claims priority toProvisional Application, Ser. No. 60/339,089, filed Dec. 10, 2001, thedisclosures of which are herein incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to microelectromechanical system (MEMS)resonators and filters, and, more particularly, to the fabrication ofsuch devices in a manner which allows integration with other integratedcircuit technologies, such as Bi-CMOS, while maintaining the desiredproperties of these devices such as high resonant frequency (f₀) andvery high quality factor (Q).

2. Description of Related Art

Microelectromechanical system (MEMS) devices have the potential forgreat impact on the communications industry. MEMS RF switches,oscillators (resonators), filters, varactors, and inductors are a few ofthe devices that could replace large and relatively expensive off-chippassive components. It is even possible that the introduction of thesetypes of MEMS devices, particularly resonators and filters, into analogand mixed-signal integrated circuits could dramatically alter thearchitecture of current wireless communication devices. Key to suchadvancements is the ability to monolithically integrate MEMS RFcomponents with integrated circuit technologies to realize cost, size,power, and performance benefits.

MEMS resonators and filters have been under development for some time.For resonators and filters aimed at RF communications applications, thekey design factors are ability to reach the frequencies of interest(approx. 900 MHz-2 GHz), low voltage operation, small size, and veryhigh quality factor (Q). Resonators and filters developed to date havedemonstrated high Qs and reasonably small sizes, but have not achievedthe frequency or bias voltage targets required for incorporation withanalog and mixed signal circuits. Other drawbacks of current MEMSresonators and filters include incompatibility of materials, processes,and processing temperatures for integration with other IC processes,inability to scale the devices to the desired sizes because of grainsize limitations of the materials used and inability to form very smallgaps between electrodes, and failure to provide protection for the MEMSdevices from subsequent processing steps and ambient conditions andcontamination.

Typical designs of prior art MEMS resonators and filters are illustratedin FIGS. 1A-1C. FIG. 1A shows a comb-drive type MEMS filter. Stationarycombs 1 and 7 are connected via anchors 2 and 8 to input and outputelectrodes 3 and 9, respectively. Moving comb 4 is connected via anchors5 to ground plane 6. The fingers of all three comb structures aresuspended above the underlying substrate, the ground plane, and theinput and output electrodes except at the anchor points. All three combsare comprised of a conductive material, typically heavily dopedpolysilicon. The ground plane and input and output electrodes are alsoconductors typically made from heavily doped polysilicon. Duringoperation, ground plane 6 is electrically contacted to the groundpotential. The potential of moving comb 4 is also at ground. An ACexcitation, superimposed on a DC bias, is applied to input electrode 3and thus, via anchor 2, to stationary comb 1. The same DC bias isapplied to output electrode 9 and thus, via anchor 8, to stationary comb7. Because of the potential difference between the fingers of stationarycomb 1 and moving comb 4, moving comb 4 is attracted laterally towardstationary comb 1. The magnitude of this potential difference, and thusthe distance which moving comb 4 travels, is modulated by the ACexcitation. When the frequency of the exciting AC voltage closelymatches the mechanical resonant frequency f₀ of moving comb 4, theamplitude of vibration of moving comb 4 reaches a maximum that isdependent on the quality factor Q of the system. Simultaneously, thefingers of moving comb 4 and stationary comb 7 comprise a time-varyingcapacitor as the amount of overlap between the fingers of the combschanges with the movement of moving comb 4. Thus, through therelationship I=d(CV)/dt, there will also be a time-varying current whichcan be sensed electrically at output electrode 9. The magnitude of thiscurrent will also be greatest when the frequency of the exciting ACvoltage at input electrode 3 closely matches the f₀. Thus, the deviceprovides electromechanical filtering of the input signal around f₀.

FIG. 1B shows another example of a prior art MEMS filter which is aimedat achieving higher-frequency operation. Two beams 11 and 15 areconnected to ground electrodes 13 via anchors 12. Beams 11 and 15 arealso connected to one another by bridge 14. Taken alone, either beam 1for beam 15 comprises a MEMS resonator. Coupling two or more MEMSresonators together creates the MEMS filter. Beams 11, 15, and bridge 14are suspended above the underlying substrate. Ground electrodes 13, andinput and output electrodes 16 and 17 (respectively) are also suspendedabove the underlying substrate except at anchor points 12. Beams 11 and15, bridge 14, and all electrodes 13, 16 and 17 are composed of aconductive material, typically heavily doped polysilicon. Duringoperation, ground electrodes 13 are electrically contacted to the groundpotential; thus, via anchors 12, the potential of beams 11 and 15 andbridge 14 are also at ground. An AC excitation, superimposed on a DCbias, is applied to input electrode 16. The same DC bias is applied tooutput electrode 17. Because of the potential difference between them,beam 11 is attracted downward toward electrode 16. The magnitude of thispotential difference, and thus the distance which beam 11 travels, ismodulated by the AC excitation. When the frequency of the exciting ACvoltage closely matches the mechanical resonant frequency f₀ of beam 11,the amplitude of vibration of beam 11 reaches a maximum that isdependent on the quality factor Q of the system. The mechanical energyof vibration of beam 11 is transmitted via bridge 14 to beam 15. Beam 15and output electrode 17 comprise a time-varying capacitor as thedistance between the two structures changes with the movement of beam15. Thus, through the relationship I=d(CV)/dt, there will also be atime-varying current which can be sensed electrically at outputelectrode 17. The magnitude of this current will also be greatest whenthe frequency of the exciting AC voltage at input electrode 16 closelymatches the f₀. Thus, the device provides electromechanical filtering ofthe input signal around f₀.

FIG. 1C shows cross section A-A′ of prior art MEMS resonator 11 as seenin FIG. 1B. This cross section also shows the substrate 21 upon whichthe MEMS resonator or filter is constructed. This substrate is typicallysilicon (Si), although other substrates such as glass, quartz, orgallium arsenide (GaAs) have also been used. Also shown is insulatinglayer 22, typically silicon dioxide (SiO₂), used to electrically isolatethe MEMS device from the substrate and other devices. Air gap 23 can beseen in the cross section, demonstrating that beam 11 is freestandingexcept at anchor points 12. Not shown here is the sacrificial materialthat occupied gap 23 during the construction of this device, and waslater removed so that beam 11 would be free to vibrate.

One of the drawbacks of the prior art is the deposition temperature ofthe materials commonly used for construction of the MEMS device.Although various conductive materials have been used to form MEMSresonators and filters, polysilicon is the most common. Polysilicon isfrequently chosen because of its relatively high ratio of elasticmodulus (E) to density (ρ). This ratio is one of the most importantfactors in determining the resonant frequency of the device, and sincehigh frequencies are sought for RF communications applications, highratios of E/ρ are desirable. However, polysilicon must be deposited attemperatures in excess of 600° C. Furthermore, the dopant atoms, such asphosphorus, which are added to the polysilicon to make it sufficientlyconductive, frequently must be annealed at temperatures near 900° C. inorder to activate them. These temperatures are well above thetemperatures used in fabrication of the metal interconnect levels ofintegrated circuit processes. This means that prior art MEMS resonatorsand filters, if they were to be integrated in an IC process, would haveto be fabricated at the same time as the transistor devices (whichpermit higher processing temperatures). This type of process integrationis much more difficult to achieve and is very specific to the particularIC process. Thus, the process steps for formation of the MEMS devicewould likely need to be altered each time there was a change to the ICprocess, or whenever it was desired to integrate the MEMS device with adifferent IC process. A much simpler and more modular approach is tointegrate the MEMS device after all circuit processing, includinginterconnect levels, has been completed. However, this cannot be donewith prior art MEMS resonators and filters.

Another serious issue with prior art MEMS resonators and filters is theprocess by which the devices are released from the surrounding layersand substrate. The most commonly used sacrificial material (i.e., thematerial which temporarily occupies the gap region and is later removedto create the freestanding MEMS structure) in the prior art is SiO₂.This material is removed by means of etching in an aqueous bufferedhydrofluoric acid (buffer-HF) solution. This solution will also removesilicon nitride (SiN), although at a slower rate, and causes etching ofor damage too many metals. Because SiO₂ and SiN are used as insulatinglayers in integrated circuits, this release method also makes it verydifficult to integrate prior art MEMS resonators and filters with ICprocesses. Another problem with the use of aqueous buffer-HF as arelease method is the occurrence of a phenomenon known as stiction.After the sacrificial SiO₂ has been removed, the buffer-HF is rinsedaway. As the water is then removed during the subsequent drying step,the freestanding MEMS parts have a tendency to stick to the substrate orsurrounding materials because of the high surface tension of the water.Prior art MEMS devices frequently have to be subjected to an alternativedrying method such as the use of supercritical carbon dioxide (CO₂).This method and the associated tools are also not part of any current ICprocess flow. Another drawback to using aqueous buffer-HF to remove thesacrificial layer is that it restricts the aspect ratios and gapdimensions that can be achieved in MEMS devices. Very small gaps(tens-few hundred nanometers) cannot be formed because of limitedtransport of the etchant and etch products in and out of the gap region.Small gaps are desirable in MEMS devices because they allow the use oflower actuation voltages. Typical RF ICs use supply voltages of 3V; mostprior art MEMS resonators and filters require biases of 20V and up.

Another concern with prior art MEMS resonators and filters is the lackof adequate encapsulation of the devices for protection duringsubsequent processing steps, and from ambient contamination, humidity,and pressure when fabrication is complete. Once the MEMS device has beenreleased, additional processing steps create the risk of re-filling thegap area with deposited material and re-connecting the device to thesubstrate, causing failures due to stiction, or adversely affectingyield or performance via the introduction of particulates to the gapregion or the device itself. Even after all fabrication is complete,MEMS resonators and filters are quite sensitive to ambient conditions.For example, a particulate adhering to the resonator beam could changethe mass (and thus the resonant frequency) of a small beam by severalhundred percent. A particulate lodged in the gap region would damp orcompletely prevent resonance. Finally, it has been established that thequality factor of MEMS resonators and filters is directly related toambient pressure, and in order to maximize Q, MEMS resonators andfilters must be operated at pressures below about 0.1 Torr. Severalencapsulation schemes have been proposed in the prior art. The mostcommon methods involve bonding a second substrate with an etched cavityover the MEMS device by various means (e.g. anodic bonding, eutecticbonding, etc.). However, to date these methods have not been adequatelydemonstrated at wafer scale. Each individual device must be capped. Thismethod is not compatible with reasonable manufacturing processes. Thismethod also causes difficulties with integrated circuit designs intendedfor packaging via flip-chip (solder bump) die attach. Furthermore, thismethod assumes that the MEMS resonator or filter is the last devicefabricated (i.e., it is exposed on the top surface of the chip), and ithas already been seen that prior-art MEMS resonators and filters are notcompatible with fabrication after the completion of IC processing.Another encapsulation method that has been proposed in the prior art isto cover the MEMS resonator with additional SiO₂, then to cap the entirestructure with a shell of porous polysilicon. The device is then exposedagain to aqueous buffer-HF, which is transported through the porouspolysilicon, removes the covering SiO₂, and diffuses back out throughthe porous polysilicon. This method is unsatisfactory for many reasons,several of which (deposition temperature of polysilicon and stiction)have already been discussed.

BRIEF SUMMARY

Accordingly, it is an object of the present application to provide aMEMS resonator or MEMS filter having electrodes energized by an appliedDC potential and excited by an applied AC potential, causing a moveablestructure to vibrate at its mechanical resonant frequency, therebyproviding a frequency reference or filtering a signal of interest aroundthe resonant frequency.

It is another object to provide a method for the construction of thesedevices which allows them to be fabricated at temperatures low enough tobe compatible with the metal interconnect levels of any analog, digital,or mixed signal integrated circuit process.

It is yet another object to provide a method for releasing thefreestanding portions of these devices from the surrounding substrateand materials in a manner which is also compatible with the metalinterconnect levels of any analog, digital, or mixed signal integratedcircuit process, which eliminates stiction during processing, and whichallows for the construction of ultra-small gaps between electrodes inthese devices.

It is a further object of the invention to provide a method ofencapsulation of these devices which protects them from subsequentprocessing steps, contamination, and ambient conditions such ashumidity, pressure, and the like.

The present invention describes a process for the fabrication of MEMSresonators or filters at temperatures low enough to permit integrationwith analog, digital, or mixed-signal IC processes after or concurrentlywith the formation of metal interconnects in those processes.

According to the present invention, the performance requirements of MEMSresonators and filters, in particular high frequency operation and highQ, can be achieved through the selection of materials that arecompatible both with IC processes and with these specifications.

The invention also includes methods for the clean removal of sacrificialmaterial without yield loss due to stiction, without need fornon-standard processing techniques, and permitting the construction ofvery small gaps that allow operation of the MEMS resonators and filtersusing actuation voltages compatible with analog IC supply voltages.

It is a principal object of the present invention to provide a method ofvacuum encapsulation of these devices that will protect them fromsubsequent processing steps (such as dicing, die attach, bonding,packaging, and the like) and ambient contamination and humidity whichmay cause device failure, and that will provide low enough pressure formaximum device performance (high Q). This method of encapsulation iswafer-scale; that is, all MEMS devices on a wafer are protectedsimultaneously during processing.

Further and still other objects of the present invention will becomemore readily apparent from the following detailed description taken inconjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1A-1C describe the prior art.

FIGS. 2A-2B show cross-sectional views of sample substrates upon whichthe device could be fabricated.

FIGS. 3-11B show cross sectional views of the device at various stagesof fabrication.

FIG. 12 shows an alternative embodiment in which the MEMS resonator orfilter has been incorporated into the interconnect layers rather thanadded afterward.

FIGS. 13A-D show an alternative embodiment in which the MEMS resonatoror filter has been fabricated on a planar surface rather than in acavity and the encapsulation provided subsequently.

FIGS. 14A-C show an alternative embodiment in which the cavity materialand sacrificial encapsulating material are shared.

DETAILED DESCRIPTION

FIGS. 3-11B show cross-sectional views according to a preferredembodiment of the invention for a device such as the one shown in theprior art in FIG. 1B and in cross-section in FIG. 1C. However, MEMSresonators and filters can be designed in a wide variety ofconfigurations including comb-drive resonators, beams fixed at two ends,beams fixed at one end, beams with suspensions, coupled beams, tuningforks, beams with bends or turns, curved beams, disks, and so on. Thesedrawings of FIGS. 3-11B are in no way intended to exclude othergeometries and configurations for building a MEMS resonator or filter,and all such geometries and configurations are included herein. Theprocess of manufacture described is the same for all suchconfigurations, and the design shown in FIGS. 3-11B is chosen forconvenience of description only.

FIG. 2A shows a typical starting substrate 31 covered by an insulatinglayer 32. Starting substrates 31 may include a variety of materials suchas silicon with or without epitaxial layers, high resistivity silicon(HRS), silicon-on-insulator (SOI), glass, quartz, sapphire, GaAs, orother substrates commonly used in integrated circuit manufacturing. Theinsulating layer 32 could be SiO₂, SiN, silicon oxy-nitrite (SiON), orany of a variety of organic insulators, or some combination of layers ofinsulating materials. If the starting substrate 31 is itself aninsulator such as quartz, the insulating layer 32 may be omitted. FIG.2B shows a semi-conducting substrate 31 such as silicon, SOI, or thelike in which active devices 33 such as transistors, diodes, varactors,etc. have already been fabricated, along with any desired localinterconnect layers 34. An insulating layer 32 has covered the substrateand active devices. The drawings that follow assume that a substratesuch as the one in FIG. 2B has been used, since this type of substrateoffers maximum utility for the MEMS device.

FIG. 3 shows the device after the completion of the fabrication of themetal interconnects used to join active and passive devices in anintegrated circuit. Interconnecting wires 41 may be made of Al, AlCu,Cu, W, or any conductive material or combination of such materialscommonly in use in IC fabrication. Inter-level vias 43 are similarlycomprised. Inter- and intra-level dielectrics (ILD) 42 may be comprisedof SiO₂, SiN, SiON, any of a variety of organic insulators, or otherinsulating material or combination of such materials commonly in use inIC fabrication. The number of layers of interconnecting wires 41,inter-level vias 43, and ILD layers 42 shown in FIG. 3 is arbitrary. Anynumber of layers, including none, may be used. These layers may bedeposited and patterned according to any method commonly in use in ICfabrication.

A final conducting layer 44 is then deposited and patterned. Conductinglayer 44 will form the input and output electrodes of the electricalconnection to, and the physical anchor points for the MEMS resonator orfilter. Conducting layer 44 may or may not be also used as aninterconnect level in the integrated circuit. Conducting layer 44 may becomprised of Al, AlCu, Cu, W, or any conductive material or combinationof such materials commonly in use in IC fabrication, with the caveatthat if a material such as Cu that oxidizes readily is used, conductinglayer 44 should have a relatively non-reactive conductive materialcoating its top surface. As long as the base or coating material willdevelop less than a few 10s of nm of oxide in the presence of an oxygenplasma, it will be satisfactory for the operation of the MEMS resonatoror filter. Otherwise, the coating material should be a noble metal suchAu, Pt, Pd, Ir, Rh, or Ru. Conducting layer(s) 44 may be deposited byany means commonly used in IC fabrication, including but not limited tosputter deposition, CVD, PECVD, evaporation, or electroplating, as longas that deposition method does not exceed the maximum allowabletemperature T_(max) which existing interconnect layers 41, vias 43, andILD 42 can withstand. Similarly, the intra-level dielectric surroundingconducting layer 44 should not be etched by an oxygen plasma; if this isthe case then this intra-level dielectric should be coated with anotherinsulating material such as SiO₂, SiN, SiON, or the like. In thepreferred embodiment, the wafer surface is planar after the constructionof conducting layer 44. This planarization may be achieved throughchemical-mechanical polishing (CMP) or other method commonly in use forIC manufacturing.

FIG. 4 shows the formation of a cavity 51 in which the MEMS resonator orfilter will be constructed. The cavity material 52 is first deposited bya means such as plasma-enhanced chemical vapor deposition (PECVD) orother method (e.g. sputtering, spin-on, etc.) that keeps the temperaturebelow T_(max). In the preferred embodiment, the cavity material shouldbe a layer or set of layers of an insulator which will not be etched byan oxygen plasma, such as SiO₂, SiN, SiON, or the like. The cavity 51 ispatterned by any typical method, such as reactive ion etching (RIE) orwet chemical etching.

In FIG. 5, a layer of sacrificial material 61 is deposited and patternedto expose anchor points 62. The space occupied by sacrificial layer 61will later form the gap between the input or output electrodes and theresonating member of the MEMS resonator or filter. As such, sacrificiallayer 61 should be made the same thickness as the desired gap spacing.We propose the use of carbon-based materials that can be easily removedin an oxygen-based dry chemistry or alternatively by annealing in thepresence of oxygen (O₂) gas at temperatures less than 400° C. (typicalT_(max)). As carbon is readily removed with O2 plasma ashing or O₂annealing, no aqueous solutions are necessary. Thus, concerns aboutstiction are alleviated. Additionally, in the O₂ ashing or annealingenvironment, most materials (with the exception of carbon-basedmaterials) do not exhibit any significant etch rates. Therefore, the useof carbon-based release layers will allow for a greater flexibility ofmaterial choices for MEMS devices. The carbon-based release layer 61 canbe deposited by a variety of methods, including but not limited toPECVD, evaporation, sputtering, and spin-on techniques. The choice ofdeposition technique generally relates to other structural requirementssuch as conformality, thickness control, and thermal stability of thesacrificial layer. The type of material can be any solid form of C, CH,CHO, or CHON. During the patterning of sacrificial layer 61 to formanchor points 62, it may be necessary or desirable to use a secondaryhard mask of metal, silicide, or other dielectric layers.

Next the materials that comprise the resonating member of the MEMSresonator or filter are deposited and patterned. In the preferredembodiment shown in FIG. 6A, the resonating member is comprised of athin layer of conductive material 70 followed by a thick structurallayer 71. Conductive layer 70 is used so that electrical contact to theresonating member may be made, and so that electrostatic actuationbetween the input or output electrode 75 and the resonator may beachieved. Because good conductors such as metals and silicides typicallyhave very high density (ρ), and low density materials are more desirablefor achieving high resonant frequencies, the conducting layer 70 is madevery thin. Any material that has good conductivity may be used, with thecaveat that if a material such as Cu that oxidizes readily is used,conducting layer 70 should have a relatively non-reactive conductivematerial coating its lower surface. As long as the base or coatingmaterial will develop less than a few 10s of nm of oxide in the presenceof an oxygen plasma, it will be satisfactory for the operation of theMEMS resonator or filter. Otherwise, the coating material should be anoble metal such Au, Pt, Pd, Ir, Rh, or Ru.

Because many dielectric materials have an excellent E/ρ ratio forachieving high frequency operation of the MEMS resonator or filter, thebulk structural material 71 of the resonating member is comprised ofdielectric in the preferred embodiment. Any material that is not etchedor significantly altered by O₂ plasma may be used; however aluminumnitride (AlN), aluminum oxide (Al₂O₃), silicon nitride (Si₃N₄), tantalumsilicon nitride (TaSiN), and many piezoelectric materials make excellentchoices. The material in layers 70 and 71 may be deposited by anytypical means whose temperature does not exceed T_(max), for examplePECVD, sputtering, evaporation, electroplating, etc. The resonatingmember is then patterned, typically by RIE.

FIG. 6B shows an alternative embodiment in which the entire thickness ofthe resonating member is made of conductive material 70. This may bedone, for example, when creating a MEMS resonator or filter whosevibration is in the lateral direction, such as a comb filter. Thedrawback to this embodiment is the lower E/ρ ratio and thus the lower f₀of the MEMS resonator or filter.

FIG. 6C shows an alternative embodiment in which the resonating memberis comprised of more than two layers of material, for example a lowerconductor 70, a structural layer 71, and an upper conducting layer 72.This may be done to offset performance effects due to the differingthermal coefficients of expansion of the different layers in theresonating member. Although only three layers are shown in FIG. 6C, anynumber of layers may be used, and any materials may be used for thedifferent layers, as long as at least one conducting layer is used, andall materials obey the processing temperature restrictions and show goodresistance to etching by oxygen plasma.

Following the deposition and patterning of the resonating member, theremainder of cavity 51 is filled in with additional sacrificial material80 (FIG. 7). This material may be the same as or different from thesacrificial layer 61 shown in FIG. 5, as long as it is also acarbon-based material which is easily removed in an oxygen plasma or byannealing in an oxygen ambient. The same material and deposition methodchoices apply. Following deposition of additional sacrificial material80, the entire structure is planarized by a method such as CMP. Thisstep may not be necessary if the material chosen for sacrificialmaterial 80 has self-planarizing properties.

In FIG. 8, the entire structure is then capped with additionaldielectric layer(s) 81. Although a single layer is shown in FIG. 8,multiple layers may be employed. This layer or layers may be comprisedof SiO₂, SiN, SiON, or the like, as long as these layers are not etchedby an O₂ plasma, and these layers may be deposited by any typical meanssuch as PECVD. Subsequently (FIG. 9), very small via holes 90 are etchedin this cavity “ceiling” by RIE, thereby exposing sacrificial material80.

Next (FIG. 10), the sacrificial material above, surrounding, and belowthe MEMS resonator or filter is removed via an O₂ ashing step, or byannealing in the presence of O₂ gas at temperatures less than T_(max).This procedure again reveals cavity 51 and creates air gap 91. Now theresonating member of the MEMS resonator or filter is free to move exceptat the anchor locations. Due to the ease of removal of the sacrificialmaterial with this process, very small gaps on the order of 100 nm canbe achieved. Since no rinsing of reagents or etch by-products isrequired, problems with stiction are eliminated.

The structure is then coated with additional dielectric layer(s) 92 asshown in FIG. 11A in a two-step process. In the first phase,non-selective PECVD is used to partially seal off release vias 90. Thepoor conformality of this process works in our favor in this case torapidly pinch off the release vias while depositing very little materialinside vias 90 or cavity 51 itself. Both the aspect ratio of releasevias 90 and the parameters of the PECVD process can be optimized tominimize deposition of unwanted material within cavity 51. In the secondphase of the process, via holes 90 are finally and completely sealed ina physical vapor deposition process such as evaporation or sputtering,wherein the ambient pressure is around 10 mT or less. This is an orderof magnitude lower than the pressure required for optimum performance ofMEMS resonators and filters. If pressure this low is not required fordevice operation, the entire pinch-off procedure can be done in a singlePECVD step. In the preferred embodiment, the release process and thepinch-off process are accomplished in the same manufacturing tool sothat the devices do not need to be exposed to the ambient in between. Ifnecessary, a forming gas anneal can be performed between these two steps(release and pinch-off) to reduce any metal oxides formed on thesurfaces of the electrodes or resonating member of the MEMS resonator orfilter. Since the material used for the pinch-off process, typicallySiO₂, SiN, SiON or some combination of these, may not provide along-term hermetic seal for the MEMS device, the alternative embodimentshown in FIG. 11B demonstrates a metal “lid” 93 used to preventdiffusion, particularly of water vapor, through the cavity ceiling. Inan alternative embodiment, the vacuum encapsulation process presentedhere could be further combined with other techniques such as eutecticbonding to another substrate, etc., to gain additional protection forthe MEMS device.

FIGS. 3-11 showed the preferred embodiment of a MEMS resonator or filterthat was fabricated after the completion of all the processing stepsrequired for IC fabrication. An alternative method is to incorporate thefabrication of the MEMS device into the process steps used for formationof the interconnect layers. For example, in FIG. 12, a MEMS device isshown wherein the metal level that forms the input and output electrodesand electrical contact/anchors for the resonating member is shared withfirst interconnect metal 101. The material that comprises the cavity isthe same as the inter-level dielectrics that insulate interconnectmetals 102 and 103. Interconnect metals 104, 105, and 106 are formedafter the completion of the fabrication and encapsulation of the MEMSresonator or filter. The number of metal levels shown in the example inFIG. 12 is arbitrary, as is the placement of the MEMS device among them.That is, fabrication of the MEMS device could have just as easily begunwith interconnect metals 102 or 103, etc.

In another alternative embodiment, the MEMS resonator or filter is notfabricated in a cavity 51 as shown in FIG. 4. Instead, the MEMSresonator or filter is first constructed on a planar surface, and thenthe encapsulation procedure is executed afterward. In FIG. 13A,sacrificial layer 61 has been deposited and patterned as in FIG. 5, andresonating structure materials 70 and 71 have been deposited andpatterned as in FIG. 6A. The difference is that cavity 51 is missing. InFIG. 13B, additional sacrificial material 80 has been deposited andplanarized as in FIG. 7. Again, however, cavity 51 is not present. InFIG. 13C, hard mask 111, typically consisting of layers of SiO₂, SiN, orSiON, has been deposited and patterned. The pattern has been transferredto sacrificial material 80. In FIG. 13D, additional dielectric material112, again typically consisting of SiO₂, SiN, SiON, or some combinationof these, has been deposited and the entire structure planarized by aprocess such as CMP. The difference between FIG. 13D and FIG. 7 is thatthe CMP process of FIG. 13D planarized an inorganic dielectric, and theCMP process of FIG. 7 planarized the carbon-based sacrificial material.Subsequent to FIG. 13D, the rest of the process is as from FIG. 8onward. The choice of which embodiment to pursue will depend on factorsincluding the lithography and CMP capabilities of the manufacturingline.

FIGS. 14A-C show an alternative embodiment in which the MEMS device isconstructed on a planar surface, and the sacrificial material is one andthe same as the “cavity” material. Assuming that the process steps whichlead up to FIG. 13B are followed, in FIG. 14A a thick dielectricmembrane 120 is deposited over the entire structure. Via holes 90 arethen etched through dielectric layer 120 (FIG. 14B). When the releaseprocess is executed, “cavity” 55 of FIG. 14C results due to theisotropic nature of the O₂ plasma etch or the anneal in O₂ containingambient. The extent of lateral etch will depend on the etch rate of thecarbon-based sacrificial material and the time required to free the MEMSstructure. The lateral etch will not be problematic as long asappropriate design ground rules are enforced in laying out the design.

Although the process described herein was developed particularly for thefabrication of MEMS resonators and filters, it should be noted that themethods, particularly of release and encapsulation, could be equallywell applied to other types of MEMS devices. For example, device shownin cross section in FIG. 11A could function as a metal-metal contactswitch (MEMS switch) if the applied DC voltage between the resonatingmember and the input or output electrode were sufficiently high.

The preferred embodiment used carbon-based release layers and an O₂plasma release process for reasons stated earlier. However, the combinedrelease-and-encapsulation process could also be applied to othermaterial sets, as long as compatibility requirements are met. The basicprocess involves forming a cavity in a material, filling the cavity witha material readily removable without significant etching of the materialsurrounding the cavity, capping the cavity with another material notreadily removed when removing the material inside the cavity, patterningsmall holes in the material capping the cavity, removing the materialwithin the cavity through the holes in the capping material, and finallysealing the cavity with a vacuum coating process. Another sacrificialmaterial that could be used with a metal/dielectric MEMS resonator orfilter is sputtered or evaporated silicon, removed with a plasmacontaining xenon difluoride (XeF₂). Other combinations are possible aswell.

While the presented invention has been described in terms of a preferredembodiment, those skilled in the art will readily recognize that manychanges and modifications are possible, all of which remain within thespirit and the scope of the present invention, as defined by theaccompanying claims.

1. A microelectromechanical system (MEMS) resonator or filtercomprising: a first conductive layer; one or more electrodes patternedin said first conductive layer which serve the function of signal input,signal output, or DC biasing, or some combination of these functions; anevacuated cavity; a resonating member comprised of a lower conductivelayer and an upper structural layer; a first air gap between saidresonating member and one or more of said electrodes; an upper membranecovering said cavity; and a second air gap between said resonatingmember and said upper membrane.
 2. The MEMS resonator or filter asrecited in claim 1, wherein a DC potential difference is applied betweensaid resonating member and one or more of said electrodes.
 3. The MEMSresonator or filter as recited in claim 1, wherein an electrostaticattraction between said resonating member and one or more of saidelectrodes causes bending or displacement of said resonating member. 4.The MEMS resonator or filter as recited in claim 1, wherein an appliedAC voltage to either said resonating member or to one or more of saidelectrodes modulates said displacement and causes vibration of saidresonating member.
 5. The MEMS resonator or filter as recited in claim1, wherein said first conductive layer is made from Al, AlCu, Cu, W, Au,Pt, Pd, Ir, Rh, Ru, TiSi, TaSi, WSi, or CoSi, conductive oxides ornitrides thereof, with or without adhesion layers such as Ti, TiN, orTaN, or some combination of these.
 6. The MEMS resonator or filter asrecited in claim 1, wherein said first conductive layer is furtherpassivated by a noble metal such as Au, Pt, Pd, Ir, Rh, or Ru.
 7. TheMEMS resonator or filter as recited in claim 1, wherein said firstconductive layer is also used as a metal interconnect level in anintegrated circuit.
 8. The MEMS resonator or filter as recited in claim1, wherein said first conductive layer is also used as a localinterconnect level in an integrated circuit.
 9. The MEMS resonator orfilter as recited in claim 1, wherein said evacuated cavity is formedwithin a layer or layers of dielectric such as SiO₂, SiN, SiON, SiCH,SiCOH, or some combination of these.
 10. The MEMS resonator or filter asrecited in claim 1, wherein said evacuated cavity is formed within anarbitrary number of existing layers of inter-level dielectric in anintegrated circuit process.
 11. The MEMS resonator or filter as recitedin claim 1, wherein said evacuated cavity is formed within a layer ofsacrificial material to be removed at the time of cavity evacuation. 12.The MEMS resonator or filter as recited in claim 1, wherein saidevacuated cavity is formed before the creation of the resonating member.13. The MEMS resonator or filter as recited in claim 1, wherein saidevacuated cavity is formed subsequent to the creation of the resonatingmember.
 14. The MEMS resonator or filter as recited in claim 1, whereinsaid resonating member is comprised entirely of said lower conductivelayer.
 15. The MEMS resonator or filter as recited in claim 1, whereinsaid resonating member is comprised of a said lower conductive layer, amiddle structural layer, and an upper conductive layer.
 16. The MEMSresonator or filter as recited in claim 1, wherein said resonatingmember is comprised of any arbitrary number and combination of layers ofsaid lower conductive layer and said middle structural layer.
 17. TheMEMS resonator or filter as recited in claim 1, wherein said lowerconductive layer of said resonating member comprises Al, AlCu, Cu, W,Au, Pt, Pd, Ir, Rh, Ru, TiSi, TaSi, WSi, or CoSi, conductive oxides ornitrides thereof, with or without adhesion layers such as Ti, TiN, orTaN, or some combination of these.
 18. The MEMS resonator or filter asrecited in claim 1, wherein said lower conductive layer of saidresonating member is further passivated by a noble metal such as Au, Pt,Pd, Ir, Rh, or Ru.
 19. The MEMS resonator or filter as recited in claim1, wherein said middle structural layer of said resonating membercomprises from a dielectric selected from the group consisting of AlN,Al₂O₃, Si₃N₄, SiN, SiO₂, SiON, TaSiN, SiCH, SiCOH, and any of a varietyof piezoelectric materials.
 20. The MEMS resonator or filter as recitedin claim 1, wherein said first air gap is created by first depositingand then later removing a layer of sacrificial material.
 21. The MEMSresonator or filter as recited in claim 1, wherein a first sacrificialmaterial used in the creation of said first air gap is one of a group ofcarbon-based materials, in any solid form of C, CH, CHO, or CHON. 22.The MEMS resonator or filter as recited in claim 1, wherein a firstsacrificial material used in the creation of said first air gap isselected from the group consisting of diamond-like carbon (DLC),Silicon-low K (SiLK©), poly arylene ether, polyimide, and photoresist.23. The MEMS resonator or filter as recited in claim 1, wherein the saidupper membrane is selected from the group consisting of SiO₂, SiN, SiON,SiCH, SiCOH, or some combination of these.
 24. The MEMS resonator orfilter as recited in claim 1, wherein said air gap between saidresonating member and said upper membrane is created by depositing andthen later removing a layer of sacrificial material.
 25. The MEMSresonator or filter as recited in claim 24, wherein said sacrificialmaterial used in the creation of the air gap between said resonatingmember and said upper membrane is one of a group of carbon-basedmaterials, in any solid form of C, CH, CHO, or CHON.
 26. The MEMSresonator or filter as recited in claim 24, wherein said secondsacrificial material used in the creation of the air gap between saidresonating member and said upper membrane is made from diamond-likecarbon (DLC), Silicon-low K (SiLK©), polyimide, or photoresist
 27. TheMEMS resonator or filter as recited in claim 21, wherein firstsacrificial layer and a second sacrificial layer are the same material.28. The MEMS resonator or filter as recited in claim 21, wherein saidfirst sacrificial layer and sacrificial layer are different materials.29. The MEMS resonator or filter as recited in claim 1, wherein smallvia holes are formed in said upper membrane covering cavity 51 for thepurpose of removal of first and second sacrificial layers.
 30. The MEMSresonator or filter as recited in claim 1, wherein small via holes insaid upper membrane are sealed by the deposition of additional materialafter removal of first and second sacrificial layers.
 31. The MEMSresonator or filter as recited in claim 1, wherein an additional layerof thin film metal is placed over said upper membrane of said evacuatedcavity subsequent to removal of a first and second sacrificial layerthrough via holes and sealing of said via holes.